Melt-textured YBa 2 Cu 3 O x crystals have been irradiated along the c-axis with
208
Pb56+ ions corresponding to dose matching fields, B Φ = 0.5 T and B Φ = 2.0 T. Magnetization measurements were conducted along the ab plane of the samples. The strength of pinning sites was investigated by measuring magnetization hysterisis and the saturation remanent magnetization M R at several temperatures. We have found that the pinning strength was considerably enhanced after irradiation at both doses. Interestingly, the pinning strength at a Pbion irradiation which corresponds to the dose matching field B Φ = 0.5 T, was found to be significantly larger than that at the dose matching field B Φ = 2.0 T at all temperatures.